1. Field of the Invention
The present invention pertains to a travel control device for excavators and, more specifically, to an excavator travel control device for controlling a starting speed of an excavator operable with a low speed travel mode and a high speed travel mode.
2. Description of the Related Art
A crawler type hydraulic excavator, one of typical heavy construction equipments, is provided with a travel control device as schematically illustrated in FIG. 1. The travel control device includes a couple of main fluid pumps 101, 102 each rotatingly driven by an engine 103 to discharge hydraulic flows. The hydraulic flows thus discharged are supplied to left and right travel motors 105, 106 under a control of travel control spools 111, 112 of a main control valve block 110 so that the travel motors 105, 106 can be rotated to move the excavator forwards or rearwards. A manual travel control device 120 is provided to control the pressure and flowing direction of a hydraulic flow discharged by an auxiliary pump 104, thus generating travel control pilot pressures (X1, X2, Y1, Y2) which are to be applied to and shift the travel control spools 111, 112.
Speed changing mechanisms 107, 108 are respectively attached to the left and right travel motors 105, 106 for lowering or raising the rotating speeds of the travel motors 105, 106. The speed changing mechanisms 107, 108 are connected to the auxiliary pump 104 via a high speed signal line 117 and serves to increase the rotating speed of the travel motors 105, 106 upon receipt of a high speed signal pressure from the auxiliary pump 104. A high speed shifting valve 116 is provided on the high speed signal line 117 to open or close the high speed signal line 117 as a selection switch 115 is turned on or off.
Depending on the kinds of hydraulic excavators, the travel speed is controlled not merely by two modes, i.e., a low speed mode and a high speed mode, but by three modes, i.e., a fixed low speed travel mode, a fixed high speed travel mode and an automatic high speed travel mode.
The excavator is always started and stopped at a low speed in the fixed low speed travel mode but the starting and stopping occurs at a high speed in the fixed high speed travel mode. In the automatic high speed travel mode, the excavator normally moves at the high speed and is automatically shifted down to the low speed to produce an increased torque if the excavator encounters a great travel load. As soon as the travel load is reduced to below a predetermined value, the excavator returns back to the high speed travel condition.
One conspicuous drawback of such travel system is that, even though an operator so wishes, it is intrinsically impossible to make slow start in the fixed high speed travel mode and the automatic high speed travel mode.
The operator usually drives the excavator faster in case where the excavator has to be moved long distance from one working place to another. In the event that there exists a need to finely control the movement of the excavator, as in case of conducting a work with a bulldozer blade while moving the excavator back-and-forth, the excavator should be moved slowly.
If the operator who has an intention to move the excavator slowly makes a mistake of starting the excavator in one of the fixed high speed travel mode and the automatic high speed travel mode, the excavator is rushed forward against the operator's intention, thus embarrassing the operator and sometimes causing an accident due to the uncontrollable starting speed. This is particularly true in a caterpillar vehicle such as a crawler type hydraulic excavator.